Many individuals suffer from heart disease caused by a partial blockage of the blood vessels that supply the heart with nutrients. More severe blockage of blood vessels in such individuals often leads to hypertension, ischemic injury, stroke, or myocardial infarction. Typically vascular occlusion is preceded by vascular stenosis resulting from intimal smooth muscle cell hyperplasia. The underlying cause of the intimal smooth muscle cell hyperplasia is vascular smooth muscle injury and disruption of the integrity of the endothelial barrier and the underlying extracellular matrix. The overall disease process can be termed a hyperproliferative vascular disease because of the etiology of the disease process. Under normal circumstances, the cells of the arterial wall can be looked at as being under stringent negative control and in a quiescent non-proliferating state, probably the consequence of contact with their specialized extracellular matrix. Desquamation of the endothelium, resulting in exposure of and possible disruption of the integrity of the extracellular matrix surrounding the cells, leads to 1) a shift in smooth muscle phenotype from a quiescent, contractile state to a migrating, proliferative form [Manderson, J. A., Arterio 9: (3) (1989)], 2) eventual migration of transformed smooth muscle cells from the medial layer to the sub-lesion intimal layer [Clowes, A. W., Circ. Res. 56: 139 (1985)] and 3) subsequent massive proliferation of the intimal smooth muscle layer resulting in arterial luminal blockage [Clowes, A. W., J. Cardiovas. Pharm. 14 (Suppl 6): S12 (1989)]. Investigations of the pathogenesis of intimal thickening have shown that, following arterial injury, platelets, endothelial cells, macrophages and smooth muscle cells release paracrine and autocrine growth factors (such as platelet derived growth factor, epidermal growth factor, insulin-like growth factor, and transforming growth factor) and other cytokines that result in the smooth muscle cell proliferation and migration. T-cells and macrophages also migrate into the neointima. [Haudenschild, C., Lab. Invest. 41: 407 (1979); Clowes, A., Circ. Res. 56: 139 (1985); Clowes, A., J, Cardiovas. Pharm. 14 (Suppl. 6): S12 (1989); Manderson, J., Arterio. 9: 289 (1989); Forrester, J., J. Am. Coll. Cardiol. 17: 758 (1991)]. This cascade of events is not limited to arterial injury, but also occurs following injury to veins and arterioles.
Vascular injury causing intimal thickening can be broadly categorized as being either biologically or mechanically induced. Atherosclerosis is one of the most commonly occurring forms of biologically mediated vascular injury leading to stenosis. The migration and proliferation of vascular smooth muscle plays a crucial role in the pathogenesis of atherosclerosis. Atherosclerotic lesions include massive accumulation of lipid laden "foam cells" derived from monocyte/macrophage and smooth muscle cells. Formation of "foam cell" regions is associated with a breech of endothelial integrity and basal lamina destruction. Triggered by these events, restenosis is produced by a rapid and selective proliferation of vascular smooth muscle cells with increased new basal lamina (extracellular matrix) formation and results in eventual blocking of arterial pathways. [Davies, P. F., Artherosclerosis Lab. Invest. 55: 5 (1986)].
Until recently, it was generally believed that this proliferation resulted from growth factors released from platelets deposited on the newly exposed matrix surface. However, recent data suggests that this phenomena occurs as a consequence of an intimate interplay between at least three components of the extracellular matrix which act strongly to influence smooth muscle cell phenotype and/or response. These components include: 1) matrix collagen and its subtypes, 2) matrix bound growth factors such as fibroblast growth factor (FGF) and transforming growth factor-.beta. (TGF-.beta.), and 3) the matrix bound proteoglycans, predominantly those containing heparan sulfate glycosaminoglycan chains.
Mechanical injuries leading to intimal thickening result following balloon angioplasty, vascular surgery, transplantation surgery, and other similar invasive processes that disrupt vascular integrity. Intimal thickening following balloon catheter injury has been studied in animals as a model for arterial restenosis that occurs in human patients following balloon angioplasty. Clowes, Ferns, Reidy and others have shown that deendothelialization with an intraarterial catheter that dilates an artery injures the innermost layers of medial smooth muscle and may even kill some of the innermost cells. [Schwartz, S. M., Human Pathology 18: 240 (1987); Fingerle, J., Arteriosclerosis 10: 1082 (1990)] Injury is followed by a proliferation of the medial smooth muscle cells, after which many of them migrate into the intima through fenestrae in the internal elastic lamina and proliferate to form a neointimal lesion.
Vascular stenosis can be detected and evaluated using angiographic or sonographic imaging techniques [Evans, R. G., JAMA 265: 2382 (1991)] and is often treated by percutaneous transluminal coronary angioplasty (balloon catheterization). Within a few months following angioplasty, however, the blood flow is reduced in approximately 30-40 percent of these patients as a result of restenosis caused by a response to mechanical vascular injury suffered during the angioplasty procedure, as described above. [Pepine, C., Circulation 81: 1753 (1990); Hardoff, R., J. Am. Coll. Cardiol. 15 1486 (1990)].
It has been shown that heparin inhibits smooth muscle cell growth both in culture and in vivo. [Tiozzo, R., Arzneim. Forsch./Drug. Res. 39: 15 (1989)]; [Clowes, A. W., Circ. Res. 58 (6): 839 (1986); Clowes, A. W., Circ. Res. 56: 139 (1985)]. As early as 1977, Clowes and Karnovsky [Clowes, A. W., Nature 265: 625 (1977)] showed that administration of commercial heparin to animals whose carotid arteries have been injured in order to produce a myointimal plaque dramatically reduced the size of the myointimal thickening. The authors, showed that the effect of heparin on the injured arterial wall was to inhibit the growth of smooth muscle cells and that this effect was, in no way, related to the anti-coagulant activity of the heparin. Heparin, through its obligatory role in promoting growth factor binding, also has been shown to promote endothelial growth, a necessary element of vascular healing following vascular injury. [Bjornsson, M., Proc. Natl. Acad. Sci. USA 88: 8651 (1991); Klagsburn, M., Cell 67: 229 (1991); Klein-Soyer, C., Arterio. 9: 147 (1989); Lindner, V. J. Clin. Invest. 85: 2004 (1990); Ornitz, D. M., Molecular and Cellular Biology 12: 240 (1992); Saksela, O., J. Cell Biol. 107: 743 (1988); Thornton, S. C., Science 222: 623 (1983)]. De Vries has also reported efficacious results in preventing restenosis in clinical studies with heparin [Eur. Heart J. 12 (Suppl.): 386 (1991)], however, Lehmann reported that chronic use of heparin (1000 units/day, s.c.) after successful coronary angioplasty paradoxically appears to increase the likelihood of restenosis, and caused abnormal bleeding in 41% of patients in the study. [JACC 17(2): 181A (1991)].
Rapamycin, a macrocyclic triene antibiotic produced by Streptomyces hygroscopicus [U.S. Pat. No. 3,929,992] has been shown to prevent the formation of humoral (IgE-like) antibodies in response to an albumin allergic challenge [Martel, R., Can. J. Physiol. Pharm. 55: 48 (1977)], inhibit murine T-cell activation [Staruch, M., FASEB 3: 3411 (1989)], prolong survival time of organ grafts in histoincompatible rodents [Morris, R., Med. Sci. Res. 17: 877 (1989)], and inhibit transplantation rejection in mammals [U.S. Pat. No. 5,100,899]. Rapamycin has also has been shown to inhibit proliferation of vascular smooth muscle cells in vitro in response to mitogenic and heterotrophic factors, and in vivo following balloon catheterization of the carotid artery. [Morris, R., J. Heart Lung Transplant. 11 (pt. 2): 1992)].